A research program is in progress dealing with several aspects of the metabolism and transport of vitamin A and of vitamin D. This program seeks to acquire detailed and fundamental information about the metabolism of these fat-soluble vitamins, and to apply this information where possible to the study of human biology and disease. Four major projects are in progress. Project One, on vitamin A transport and retinol-binding protein (RBP) metabolism, aims to examine the plasma vitamin A transport system in detail, with the goal of defining the cellular and molecular mechanisms involved in the regulation of RBP synthesis and secretion by the liver. Studies with differentiated rat hepatoma cells in culture in vitro have indicated that these cell lines represent good models with which to explore these mechanisms. Studies of RBP synthesis in vitro, using isolated rat liver messenger RNA and rabbit reticulocyte lysate translation system, are in progress. Project Two aims to characterize the enzymatic hydrolysis of retinyl esters in rat liver and to explore the metabolic regulation of retinyl ester hydrolase activity in liver. Information has been obtained about many of the characteristics of retinyl palmitate hydrolase activity in liver homogenates. Studies are in progress aimed at the purification and further characterization of this activity. Project Three aims to examine in detail the soluble intracellular binding proteins for retinol and for retinoic acid. A specific radioimmunoassay for rat cellular retinol-binding protein (CRBP) was developed, and was applied to the quantitative study of the tissue distribution and the subcellular localization of CRBP in rats. Project Four focuses on the protein, called DBP, responsible for the transport of vitamin D and its metabolites in human plasma. The effects of several different protein modification procedures on the interaction between DBP and 25-hydroxy-vitamin D3 were explored in detail.